Deserialize from JSON back to object model

[dougsatch]

As part of some integration tests, I serlialize the json to file, and then want to deseriliaze back to the object model.

I had to make some minor code changes, by creating no-arg constructors for MandrillRecipient, MergeVar and MandrillAttachment.

Would you be able to make those small changes to master - it would really help alot.

Thanks for the great work!